Taxon-Specific Habitat and Tidal use by Birds in an Oyster Culture Estuary

Abstract

Shorebirds use a variety of intertidal estuarine habitats to rest and refuel during their seasonal migrations. Birds can be found foraging on mud or sandflats, aquatic vegetation, as well as intertidal areas developed for shellfish aquaculture. In Washington State, which contributes substantially to commercial U.S. production of the Pacific oyster (Crassostrea gigas), little research has been published about how aquaculture habitats are used by shorebirds relative to surrounding seagrass and mudflat. Using photographic sampling, shore- and waterbird use of mudflat, seagrass, and longline oyster culture habitats was studied on an oyster farm in Grays Harbor, WA. The effect of the tidal stage (ebb, dry, or flood periods) was also evaluated. Thirteen bird taxa were identified and analyzed for effects of habitat on community composition and total bird abundance, whereas the six most common taxa were used in an analysis of habitat type and tidal stage effects on taxon abundance. Of the six focal taxa, black-bellied plover, American crow, and dunlin (Pluvialis squatarola, Corvus brachyrhynchos, and Calidris alpina, respectively) responded significantly to habitat type - having positive associations with eelgrass and/or longlines - whereas dunlin, dowitcher, and gulls (C. alpina, Limnodromus spp., and Larus spp., respectively) responded significantly to tidal stage - having positive associations with the ebb or flood periods. Total bird observations varied by habitat and through the tidal cycle, where more birds were observed in eelgrass and during ebb and flood periods. There was no strong effect of habitat type on community composition when sampling across several months. Overall, all three habitat types were used by a variety of shore- and waterbird taxa, with no evidence of a negative effect of longline oyster culture on bird abundance.